β-galactosidase (EC 3.2.1.23) is an enzyme which hydrolyzes a β-D-galactoside linkage to release D-galactose, and β-galactosidase isozymes are generally found in a wide range of microorganisms and plants. β-galactosidase is otherwise referred to as lactase. β-galactosidase is also capable of transferring a galactoside linkage, and there are known methods in which this capability is used to produce galacto-oligosaccharides (oligosaccharides having galactose residues).
Galacto-oligosaccharides, which are linear or branched polymers of galactose, are composed of galactose except that they terminate in a glucose unit. Galacto-oligosaccharides are contained in mother's milk, which is reported to be a mixture of a variety of galacto-oligosaccharides (Non Patent Literature 1).